The present invention relates to novel thioester derivatives of the general formula (I) prepared by the reaction of 4-halogeno-2-methoxyimino-3-oxo-butyric acid (II) with 5-substituted-1,3,4-oxadiazole-2-thiol of formula (III). The invention also discloses the use of the new intermediate (I) for the preparation of cephalosporanic antibiotics (VI) in excellent yields and purity. 
wherein
X represents halogen (Cl,Br and I)
R1 represents C1-C4 alkyl or phenyl
Acid chlorides, anhydrides, esters, amide etc. are reported in the chemical literature for activation of carboxylic acid of formula (IV). Activation in the form of acid chloride required protection and deprotection of NH2 group. 
Activation of 2-(2-aminothiazol-4-yl)-2-methoxyiminoacetic acid (IV) by SO2Cl2/DMF is reported in U.S. Pat. No. 5,856,502 and activation of SOCl2/DMF is reported in U.S. Pat. No. 5,037,988. These processes suffers with the limitation of poor by moderate yields along with the use of solvents like benzene and stringent conditions required for carrying out the reactions at commercial scale.
In U.S. Pat. Nos. 4,576,749 and 4,548,748 the acid of formula (IV) have also been activated by reacting with 1-hydroxybenzotriazole (HOBT) or 2-mercaptobenzothiazole (MBT) in the presence of dicyclohexylcarbodiimide (DCC) to produce reactive ester of the acid (IV) which reacted to cephem moiety to prepare cephem antibiotics, but the processes are time consuming and with low yields, hence not suitable.
U.S. Pat. No. 4,767,852 discloses a process for production of cephems by acylating 7-amino-3-cephem-4-carboxylic acid with 2-mercaptobenzothiazolyl-(Z)-2-(2-aminothiazol-4-yl)-2-methoxyiminoacetate (MAEM). Similarly, U.S. Pat. No. 5,026,843 disclosed a process for preparing ceftriaxone disodium hemiheptahydrate by acylation of 7-amino-3-[[(2,5-dihydro-6-hydroxy-2-methyl-5-oxo-1,2,4-triazin-3yl)thio]methyl]3-cephem-4-carboxylic acid (ACT) by using MAEM as acylating agents in good yield and quality. Thus MAEM has become the standard acylating agent for the preparation of cephalosporins having an oximino group and a 2-aminothiazolyl group in 7-position of cephem compounds.
However, the synthesis of MAEM from 2-(2-aminothiazol-4-yl)-2-methoxyiminoacetic acid (IV) and 2,2xe2x80x2-dithio-bis-benzothiazole involves use of costly condensing agent triphenylphosphine (TPP). Moreover, during condensation of MAEM with 7-amino-3-cephem-4-carboxylic acid compound (V), a toxic compound MBT is also produced as a byproduct, see e.g., Chemical Abstracts, 111, 19243p (1989) which is difficult to remove completely.
Thus it is evident that the procedures described in the prior art for preparation of these antibiotics are complex, involving protection, deprotection and are associated with toxic byproduct generation. Hence there is a need to develop new acylating agents which are capable of transferring the 2-aminothiazolyl moiety to cephem compounds of formula (V) in good yield but without producing this toxic byproduct. On the similar lines, a new thioester was reported by D. G. Walker, Tet. Lett. 1990, 31,6481 to acylate the cephem moiety to get cefepime sulfate but yields obtained by using this thioester were in the range of 54-73% which cannot be considered as good yield to operate a process at commercial scale. The use of this thioester was reported in the Tet. Lett. 1990, 31, 6481 only for cefepime and not for other cephalosporins. This thioester was exploited in U.S. Pat. No. 5,869,649 for making three other important cephem antibiotics.
Synthesis of 4-halogeno-2-methoxyimino-3-oxo-butyric acid is reported in Patent No. EP 0 030 294 and a large number of references are available in the patent literature disclosing the use of 4-halogeno-2-methoxyimino-3-oxo-butyric acid represented by formula (II) as the starting material. EP 0 030 294 and WO 00 0063214 discloses the condensation of the 4-halogeno-2-methoxyimino-3-oxo-butyric acid represented by formula (II) with cephem carboxylic acids by using PCl5 Another EP Patent No. 0 842 937 discloses the formation of amide bond with cephem moiety by reacting with the thioester derivative prepared by using 2,2xe2x80x2-dithio-bis-benzothiazole. The preparation of this active thioester involves use of same costly condensing agent triphenylphosphine (TPP) which has been mentioned earlier in the text. Broadly the use of 4-halogeno-2-methoxyimino-3-oxo-butyric acid represented by formula (II) also suffer with almost in same disadvantages which are commonly prevalent for 2-(2-aminothiazol-4-yl)-2-methoxyiminoacetic acid (IV).
The primary objective of the invention is to provide new reactive thioester derivatives of 4-halogeno-2-methoxyimino-3-oxo-butyric acid of the general formula (I), which would be suitable for being used in the manufacture of cephalosporin antibiotics and would not be associated with the complexities mentioned above.
Another objective of the present invention is to provide a process for the preparation of above mentioned new thioesters (I) in good yields.
One more objective of the present invention is to provide a process for the preparation of cephalosporin antibiotics of the general formula (VI) from the said novel thioester derivatives.
Another objective of the present invention is to provide a process for the preparation of cephalosporin antibiotics e.g., cefotaxime, ceftriaxone, cefetamet, ceftiofur, cefpodoxime etc. which comprises condensation of new reactive derivatives (I) with cephem compounds (V) and in situ cyclisation with thiourea to obtain targeted antibiotics(VI) in excellent yields and purity.
Still another objective of the present invention is to produce cephalosporin antibiotics that are highly pure and free from toxic byproducts.
The present invention provides novel thioester derivatives of 4-halogeno-2-methoxyimino-3-oxo-butyric acid of the general formula (I) also, the invention provides a method by which the said thioester derivatives can be prepared by reacting of 4-halogeno-2-methoxyimino-3-oxo-butyric acid of the general formula (II) with 2-mercapto-5-substituted-1,3,4-oxadiazole of the general formula (III) (preparation of III, J. Am. Chem. Soc., 1955, 77, 400) by activating with DMF/POCl3 in presence of an organic base in a solvent. The so obtained thioester derivatives are reacted with 7-amino-cephem carboxylic acids of the general formula (V) to produce cephalosporin antibiotic compounds having the general formula (VI).
The present invention provides new thioesters of 4-halogeno-2-methoxyimino-3-oxo-butyric acid of general formula (I). The synthesis of compound (I) is achieved by preparing activated complex of 4-halogeno-2-methoxyimino-3-oxo-butyric acid (II) with DMF-POCl3 followed by the reaction with thio-oxadiazoles of the general formula (III) in organic solvent in presence of an organic base at the temperature between xe2x88x9230xc2x0 C. and +20xc2x0 C. The reactive active ester is obtained quantitative yields (95-99%). 
wherein
X represents halogen
R1 represents C1-C4 alkyl or phenyl
The reactive thioester were characterized by NMR, IR and Mass spectra. 
A major side product (VII) which is formed during this reaction has also been controlled in the process. Surprisingly this side reaction has never been mentioned in the literature.
In an embodiment, in the compound of formula (I), X is chloro, bromo or iodo.
In another embodiment the organic solvent is selected from the group comprising dichloromethane, tetrahydrofuran, dioxane, N,N-dimethylformamide, acetonitrile and mixtures thereof.
In still another embodiment the organic base is selected from the group comprising triethylamine, diethylamine, tributylamine, pyridine, N-alkylanilines, and mixtures thereof.
The compound (I) so obtained is reacted with 7-amino cephem carboxylic acid of general formula (V) in two different methods and both the methods lead to same product with comparable yields and purity.
Using above mentioned thioester the cephalosporin antibiotics obtained are of high purity (90-99%). The method gives an excellent yield (70-95%) of cephalosporin without necessitating the protection of the amino group of the acylating agents, and the toxic byproduct 2-mercaptobenzothiazole is not produced.
The cephalosporin antibiotic were synthesized by following two methods:
Method -I
The reactive thioester (I) was reacted with 7-aminocephem compound (V) 
wherein
R1 represents C1-C4 alkyl or phenyl
R2 represents H, CH3, CH2OCH3, CH2OCOCH3, 
or a standard cephalosporin substituent
R3 is hydrogen, salt or carboxylic protecting group.
R4 is hydrogen or silyl.
In organic solvent in the presence of base to obtained condensed product, which was not isolated and is directly cyclised with thiourea in mixture of water and a polar organic solvent like tetrahydrofuran, dimethylformamide, dioxane, alcohol to obtain desired cephalosporanic antibiotics of very good purity and excellent yields.
Method -II
In this approach, starting from active ester of formula (I) final product was prepared in one pot reaction. The process comprises cyclization of active ester in the first step and in same reactor addition of amino cephem compound in mixture of water and a polar organic solvent like tetrahydrofuran, dimethylformamide, dioxane, alcohols to obtain desired cephalosporanic antibiotics of equally good purity and yields as compared to first approach. This approach provides a simple, cheap and commercially viable method without the necessity of isolating thioester and without producing any toxic byproduct namely 2-mercaptobenzothiazole.
The substituent R2 in cephem compound (V) and (VI) represents hydrogen, methyl, acetyloxymethyl, methoxymethyl, 2,5-dihydro-6-hydroxy-2-methyl-5-oxo-1,2,4-triazine-3-thiol, furanyl-2-carbonyl thiol or a standard cephalosporin substituents.
R3 in cephem compound (V) and (VI) represents hydrogen, salt or a ester group which can be easily removed e.g., p-methoxybenzyl, p-nitrobenzyl, diphenylmethyl, phenacyl, trimethylsilyl etc.
In an embodiment of the present invention the organic base may be selected from the group consisting of triethylamine, N-methylmorpholine, pyridine, N-methylanilines, 1,5-diazabicyclo[4.3.0] non-5-ene, 1,4-diazabicyclo[2.2.2]octane, 4-dimethylaminopyridine, and mixtures thereof.
Many other beneficial results can be obtained by applying disclosed invention in a different manner or by modifying the invention with the scope of disclosure. However, since the major characteristic feature of the present invention resides in the use of novel reactive thioester derivatives of 4-bromo-2-methoxyimino-3-oxo-butyric acid of the general formula (I) in preparing the cephalosporin antibiotics, the technical scope of the present invention should not be limited to the following examples. The following examples are provided to illustrate but not to limit the claimed invention.